Introduction Recent interest has focussed on potential regulatory roles for vitamin D in the development and course of asthma.

Aim We assessed the prevalence and characteristics of patients with vitamin D deficiency, during a calendar year, among our Regional Difficult Asthma Clinic population.

Methods Retrospective study was undertaken to assess vitamin D deficiency prevalence plus characterisation of “deficient” compared to “normal” vitamin D level patients. Characterisation included demographic factors, associated comorbidities, asthma severity (BTS steps), and spirometry during that 12 month period. Comorbidities recorded included BodyMass Index (BMI), smoking status, atopic status, rhinitis, food allergy, salicylate sensitivity, sulphite sensitivity, Gastro-oesophageal Reflux disease (GORD), dysfunctional breathing, and psychological comorbidity.

Results Serum vitamin D3 assessment was available for 85.4% (158/185) patients during the previous 12 months. First measured vitamin D3 levels were used for primary analysis showing a mean clinic vitamin D3 level of 45.1nmol/L. Normal vitamin D3 levels (>72nmol/L) occurred in 18.9%, 14.6% had insufficient levels (52–72nmol/L) and 65.5% had deficient levels (<52nmol/L). Severe deficiency (<20nmol/L) occurred in 14.6%. Vitamin D deficiency showed highest prevalence during Winter, Autumn and Spring(83–79%) and lowest level in Summer (53.3%). Two thirds of patients received a course of vitamin D replacement therapy during the year.

Vitamin D deficient showed no difference compared to normal vitamin D3 status patients in atopic status, spirometry, inhaled medication usage, maintenance oral steroid use, Omalizumab use and asthma hospitalisations. No significant difference was observed between the two groups in respect to comorbidities like GORD, rhinitis, COPD, bronchiectasis, food allergy and dysfunctional breathing. However, vitamin D deficient subjects had significantly higher BMI (31.6 v 26.4 kg/m²; p=0.007), obesity (BMI>30) (55.3% v 13%; p<0.001) and psychological comorbidity (26.7% v 3.6%; p =0.009).

Conclusion Vitamin D deficiency is highly prevalent in the Difficult Asthma group. The clinical relevance of this finding remains unclear. While vitamin D deficient patients did not show greater asthma treatment needs or healthcare use that might have reflected influence of receiving high dose vitamin D replacement. Patients with vitamin D deficiency showed significantly greater obesity and psychological comorbidity. The association of vitamin D and Difficult Asthma merits further attention.